Many polymers are not very stable to the effects of ultraviolet (UV) radiation. Exposure to UV radiation over extended periods of time causes hazing, property loss and yellowing. This lack of property stability upon exposure to UV radiation has long limited the uses of these materials in outdoor environments.
The addition of UV absorbing compounds, sometimes called UV stabilizers, are used to retard the loss of properties and particularly the development of yellow coloration in UV unstable polymers when exposed to UV radiation. The UV absorbing compounds must have an extinction coefficient significantly higher than that of the polymers to be stabilized such that almost all of the incident UV radiation is absorbed by the compounds rather than the polymers. The energy absorbed by the UV absorbing compounds is harmlessly transferred to the polymers as heat rather than transferred electronically to the polymer chains. The fragmentation of the chains is thereby suppressed and the properties of the polymers are retained for a longer time of exposure. Thus, UV unstable polymers that are protected by UV absorbing compounds may be used in applications that have exposure to UV radiation.
The UV unstable polymers protected with UV absorbing compounds are typically used in sheet or extruded profile form. Sheeting or profiles made from clear, colored or pigmented polymers are advantageously employed in a great number of outdoor applications when they have sufficient color stability, mechanical property retention and thermoformability. The back side of the sheeting or profile may also be printed on or have a decorated sheet adhered thereto. Representative applications include signs and marquees; vehicle luggage carriers; solar roof panels; skylights; highway sound barriers; greenhouse panels; aquarium walls; parts for motor and recreational vehicles such as windows, roofs, body panels, bug and air deflection screens and vents; transparent or translucent awnings; formed letters for application to buildings; airport runway marker signs; multi-wall sheeting for use in signs; facia for soft drink and juice dispensing machines; and siding for houses.
For economic reasons, the UV stabilized polymers are typically made into multi-layer structures having a UV protective layer, which is exposed to UV radiation, and an underlying substrate layer of the polymer, which is protected from the effects of UV radiation solely by the screening of harmful UV light by means of the UV protective layer. In current commercial practice, the UV protective layer contains a UV absorbing compound and a polymeric base material, which is chosen depending on the end use. Typical polymeric base materials include polyesters, polycarbonates and acrylics. The substrate layer is typically a polyester, polycarbonate, polyvinyl chloride polymer or copolymer, chlorinated polyvinylchloride, polyamide, polyetherimide or polyethersulfone. The UV protective layer is typically either laminated or co-extruded onto the substrate layer. The UV protective layer performs its protective function as a relatively thin film as compared to the substrate layer, thereby providing cost reduction by reducing the amount of UV absorbing compound needed.
The UV stabilized structures may be sheets, films, profiles, or hollow profiles depending on the final use of the UV stabilized structures. The hollow profiles are typically made by connecting two or more sheets with ribbings running the length of the sheeting. The ribbings are spaced so as to provide structural stiffness to the final structures that would otherwise not be present. The ribbings make continuous channels down the length of the structure.
In the past, there have been a number of strategies evolved to address the problems associated with UV protection. These strategies fall into two broad categories: 1) use of a UV protective layer of acrylic-based polymer containing a UV absorbing compound to protect the underlying polymeric material, and 2) use of a UV absorbing compound incorporated directly into the polymeric material, usually at a high loading and then applied in a thin layer on the surface of the underlying polymeric material.
This first category of using a protective acrylic layer usually, but not always, overcomes the deficiency of instability toward weathering of the underlying polymeric material by means of screening harmful UV radiation from the underlying polymeric material and thereby diminishing any eventual yellowing and property loss from absorbed ultraviolet light. The major problem with such an approach is the lack of a suitable technology for making a construction with this UV protective layer, which is both economical and durable over long periods of time. Because of the vast differences in rheology of the underlying polymeric material usually employed in this case from that of the acrylic material, co-extrusion is not a good option. Therefore, frequently such constructions are made, in the case of sheeting, for example, by the process of extrusion lamination. This process involves the lamination of a film of the acrylic material onto the polymeric substrate during its extrusion and tends to be more costly than a co-extrusion process, which can be used in the second category mentioned above with a heavily loaded UV layer as the co-extrusion layer. Quite often, however, the economics of the process are only part of the problem. Often a major problem with this laminated film approach is one of eventual delamination of the construction, especially in the conditions of high humidity and heat. This renders the construction totally unfit for use when such delamination occurs.
An additional problem that often overrides all of those mentioned above, is that the acrylic materials tend to be brittle by themselves and therefore tend to reduce the impact properties of the resultant laminate. This means that even the initial sheeting thus produced tends to have dramatically lowered impact resistance as compared to sheeting not having the acrylic-based UV protective layer.
The second category, that of using an UV absorbing compound incorporated directly into the polymeric material, is disclosed, for example in U.S. Pat. Nos. 5,480,926, 5,558,912, and 5,709,929. In U.S. Pat. No. 5,709,929, a multi-layered polyester sheet is produced wherein all of the layers are of the same composition, and one of the surface layers also contains a UV absorbing compound. U.S. Pat. Nos. 5,480,926 and 5,558,912 disclose the use of high levels of UV absorbing compounds in polyesters or co-polyesters in co-extrusion or solvent coated constructions. These references, however, specifically teach that only benzoxazinones are suitable UV absorbing compounds to effect not only protection of the polyester or copolyester from discoloration but also loss of impact during the weathering process. It is specifically taught that a benzotriazole-based UV absorbing compound, 2-(-2'-hydroxy-5'-t-octylphenyl)benzotriazole, was ineffective in retention of impact strength. U.S. Pat. No. 5,783,307 discloses a multi-layered polyester sheet wherein the layer containing the UV absorbing compound also contains an optical brightener to aid in detection of the layer.
The second category of direct incorporation of the UV absorber into the polymeric matrix to be protected has the deficiency that UV light is still able to damage the polymer matrix in which the UV absorbing compound is present, as the UV absorbing compound cannot preferentially absorb 100% of the incident light. That portion of the UV light not absorbed by the UV absorbing compound, even through it may be in a tiny fraction, over long exposure times contributes to damage of the polymeric matrix material in the form of eventual discoloration, usually yellowing, and also property loss, such as eventual loss of ductility in the event of an impact.
Thus, a construction is needed where the above-named deficiencies are overcome, where the UV protective layer is itself stable to the effects of weathering, unlike where the heavily loaded polyester or polycarbonate is the UV protective layer, while at the same time it can be made into a structure with long-term integrity. This structure also needs to have impact properties that are not reduced by the presence of the UV protective layer unlike what is seen with constructions having an acrylic layer as the UV protective layer